(1) Field of the Invention:
The present invention relates to a method of producing grain oriented silicon steel sheets or strips having high magnetic induction and low iron loss, and more particularly the present invention provides a method of producing grain oriented silicon steel sheets or strips having high magnetic induction and low iron loss, wherein an intermediate annealing is carried out under a particular condition based on the result on the investigation of the behavior of silicon steel sheets in the intermediate annealing as a means for improving surely, stably and advantageously the above described two magnetic properties.
(2) Description of the Prior Art:
Grain oriented silicon steel sheets are mainly used in the iron cores of a transformer and other electric instruments, and are required to have such excellent magnetic properties that the magnetic induction represented by B.sub.10 value is high and the iron loss represented by W.sub.17/50 is low.
Particularly, it is necessary to satisfy the following two requirements in order to improve the magnetic properties of grain oriented silicon steel sheets. Firstly, it is necessary to arrange the highly aligned &lt;001&gt; axis of secondary recrystallized grains in the steel sheet uniformly in the rolling direction, and secondary to make the amount of impurities and precipitates remained in the final product as few as possible.
In order to satisfy the requirements, a fundamental production method of grain oriented silicon steel sheets through a two-stage cold rolling was firstly proposed by N. P. Goss, and various improved methods thereof have been proposed, and the magnetic induction of grain oriented silicon steel sheet is higher and the iron loss thereof is lower year after year. Among the improved methods, typical methods are a method disclosed in Japanese Patent Application Publication No. 15,644/65, wherein the finely precipitated AlN is used (hereinafter, referred to the former method), and a method disclosed in Japanese Patent Application Publication No. 13,469/76, wherein a mixture of Sb and Se or Sb and S as inhibitors is used (hereinafter, referred to the latter method). In these methods, a product having a B.sub.10 value higher than 1.89 can be obtained.
It has been known that, in the former method of Japanese Patent Application Publication No. 15,644/65, wherein the finely precipitated AlN is used, a product having high magnetic induction can be obtained, but its iron loss is relatively high due to the large secondary recrystallized grains after final annealing. Recently, an improved method has been proposed in Japanese Patent Application Publication No. 13,846/79, wherein the inter-pass aging is carried out during the course of cold rollings at high reduction rate to form the secondary recrystallized grains with the small sizes and thereby to decrease the iron loss. According to this method, products having an iron loss W.sub.17/50 lower than 1.05 W/kg can be obtained. However, the iron loss is not satisfactorily low as compared with the high magnetic induction. In order to obviate the above described drawbacks, a method for decreasing the iron loss of grain oriented silicon steel sheet has quite recently been disclosed in Japanese Patent Application Publication No. 2,252/82, wherein laser beams are irradiated on the surface of a final product steel sheet at an interval of serval mm in substantially the rectangular direction with respect to the rolling direction to introduce artificial grain boundary on the steel sheet surface. However, this method for introducing the artificial grain boundary forms locally a high dislocation density area, and therefore the resulting product has such a serious drawback that the product can only be used stably under a low temperature condition of not higher than 350.degree. C.
While, the latter method of Japanese Patent Application Publication No. 13,469/76 is a method found out by the inventors. In this method also, a high magnetic induction of B.sub.10 of at least 1.89 T can be obtined. However, in order to obtain a product having a higher magnetic induction, the inventors disclosed improved methods in Japanese Patent Laid-Open Specification No. 11,108/80, wherein Mo is added to the raw material silicon steel together with Sb and one of Se and S, and in Japanese Patent Laid-Open Specification No. 93,823/81, wherein Mo is added to the raw material silicon steel together with Sb and one of Se and S, and a steel sheet heated in the intermediate annealing just before the final cold rolling is subjected to a rapid cooling treatment, whereby a grain oriented silicon steel sheet concurrently having a high magnetic induction of B.sub.10 of at least 1.92 and a low iron loss of W.sub.17/50 of not higher than 1.05 W/kg is produced. However, this method is still insufficient for producing steel sheets having a satisfactorily low iron loss.
Since the energy crisis in several years ago, it has been eagerly demanded to develop grain oriented silicon steel sheets having an ultra-low electric power loss to be used as an iron core material.
In order to accomplish advantageously the above described demand, the inventors have investigated a method for improving advantageously the magnetic properties of a grain oriented silicon steel sheet by innovating the intermediate annealing method of the steel sheet.